GPS receiver and method for processing GPS signals

ABSTRACT

A global positioning system (GPS) receiver has first circuitry for receiving and processing pseudorandom sequences transmitted by a number of GPS satellites. The first circuitry is configured to perform conventional correlation operations on the received pseudorandom sequences to determine pseudoranges from the GPS receiver to the GPS satellites. The GPS receiver also includes second circuitry coupled to the first circuitry. The second circuitry is configured to receive and process the pseudorandom sequences during blockage conditions. The second circuitry processes the pseudorandom sequences by digitizing and stoning a predetermined record length of the received sequences and then performing fast convolution operations on the stored data to determine the pseudoranges. The GPS receiver may have a common circuitry for receiving GPS signals from in view satellites and downconverting the RF frequency of the received GPS signals to an intermediate frequency (IF). The IF signals are split into two signal paths; a first of which provides the conventional correlation processing to calculate the pseudoranges. During blockage conditions, the IF signal is passed to the second signal path wherein the IF signals are digitized and stored in memory and later processed using the fast convolution operations to provide the pseudoranges. Alternative arrangements for the two signal paths include separate downconverters or shared digitizers. One embodiment provides both signal paths on a single integrated circuit with shared circuitry executing computer readable instructions to perform GPS signal processing appropriate to the reception conditions.

RELATED APPLICATIONS

This application is related to and hereby claims the benefit of thefiling date of a provisional patent application by the same inventor,Norman F. Krasner, which application is entitled Low Power, SensitivePseudorange Measurement Apparatus and Method for Global PositioningSatellites Systems, Ser. No. 60/005,318, filed Oct. 9, 1995.

FIELD OF THE INVENTION

The present invention concerns receivers capable of determiningpseudoranges to orbiting satellites and, in particular, concerns suchreceivers as find application in global positioning satellite systems(GPS).

BACKGROUND

GPS receivers normally determine their position by computing relativetimes of arrival of signals transmitted simultaneously from amultiplicity of GPS (or NAVSTAR) satellites. These satellites transmit,as part of their message, both satellite positioning data as well asdata on clock timing, so-called "ephemeris" data. The process ofsearching for and acquiring GPS signals, reading the ephemeris data fora multiplicity of satellites and computing the location of the receiverfrom this data is time consuming, often requiring several minutes. Inmany cases, this lengthy processing time is unacceptable and,furthermore, greatly limits battery life in micro-miniaturized portableapplications.

Another limitation of current GPS receivers is that their operation islimited to situations in which multiple satellites are clearly in view,without obstructions, and where a good quality antenna is properlypositioned to receive such signals. As such, they normally are unusablein portable, body mounted applications; in areas where there issignificant foliage or building blockage (e.g., urban canyons); and inin-building applications.

There are two principal functions of GPS receiving systems: (1)computation of the pseudoranges to the various GPS satellites, and (2)computation of the position of the receiving platform using thesepseudoranges and satellite timing and ephemeris data. The pseudorangesare simply the time delays measured between the received signal fromeach satellite and a local clock. The satellite ephemeris and timingdata is extracted from the GPS signal once it is acquired and tracked.As stated above, collecting this information normally takes a relativelylong time (30 seconds to several minutes) and must be accomplished witha good received signal level in order to achieve low error rates.

Virtually all known GPS receivers utilize correlation methods to computepseudoranges. GPS signals contain high rate repetitive signals calledpseudorandom (PN) sequences. The codes available for civilianapplications are called C/A codes and have a binary phase-reversal rate,or "chipping" rate, of 1.023 MHz and a repetition period of 1023 chipsfor a code period of 1 msec. The code sequences belong to a family knownas Gold codes. Each GPS satellite broadcasts a signal with a unique Goldcode.

For a signal received from a given GPS satellite, following adownconversion process to baseband, a correlation receiver multipliesthe received signal by a stored replica of the appropriate Gold codecontained within its local memory, and then integrates, or lowpassfilters, the product in order to obtain an indication of the presence ofthe signal. This process is termed a "correlation" operation. Bysequentially adjusting the relative timing of this stored replicarelative to the received signal, and observing the correlation output,the receiver can determine the time delay between the received signaland a local clock. The initial determination of the presence of such anoutput is termed "acquisition." Once acquisition occurs, the processenters the "tracking" phase in which the timing of the local referenceis adjusted in small amounts in order to maintain a high correlationoutput. The correlation output during the tracking phase may be viewedas the GPS signal with the pseudorandom code removed, or, in commonterminology, "despread." This signal is narrow band, with bandwidthcommensurate with a 50 bit per second binary phase shift keyed datasignal which is superimposed on the GPS waveform.

The correlation acquisition process is very time consuming, especiallyif received signals are weak. To improve acquisition time, most GPSreceivers utilize a multiplicity of correlators (up to 12 typically)which allows a parallel search for correlation peaks.

Some prior GPS receivers have used FFT techniques to determine theDoppler frequency of the received GPS signal. These receivers utilizeconventional correlation operations to despread the GPS signal andprovide a narrow band signal with bandwidth typically in the range of 10kHz to 30 kHz. The resulting narrow band signal is then Fourier analyzedusing FFT algorithms to determine the carrier frequency. Thedetermination of such a carrier simultaneously provides an indicationthat the local PN reference is adjusted to the correct phase of thereceived signal and provides an accurate measurement of carrierfrequency. This frequency may then be utilized in the tracking operationof the receivers.

U.S. Pat. No. 5,420,592 to Johnson discusses the use of FFT algorithmsto compute pseudoranges at a central processing location rather than ata mobile unit. According to that method, a snapshot of data is collectedby a GPS receiver and then transmitted over a data link to a remotereceiver where it undergoes FFT processing. However, the methoddisclosed therein computes only a single forward and inverse FastFourier Transform (corresponding to four PN periods) to perform the setof correlations.

SUMMARY

One embodiment of the present invention provides a global positioningsystem (GPS) receiver having first circuitry for receiving andprocessing pseudorandom sequences transmitted by a number of GPSsatellites. The first circuitry is configured to perform conventionalcorrelation operations on the received pseudorandom sequences todetermine pseudoranges from the GPS receiver to the GPS satellites. TheGPS receiver also includes second circuitry coupled to the firstcircuitry. The second circuitry is configured to receive and process thepseudorandom sequences during blockage conditions. The second circuitryprocesses the pseudorandom sequences by digitizing and storing apredetermined record length of the received sequences and then performsfast convolution operations on the stored data to determine thepseudoranges.

In one embodiment, the GPS receiver has a common antenna for receivingGPS signals from in view satellites; and a common downconverter forreducing the RF frequency of the received GPS signals to an intermediatefrequency (IF). The IF signals are then split into two signal paths. Afirst of the signal paths provides for conventional GPS signalprocessing using correlation operations to calculate the pseudoranges.During blockage conditions, the IF signal is passed to the second signalpath wherein the IF signals are digitized and stored in memory for laterprocessing in the receiver. This later processing is accomplished usinga programmable digital signal processor which executes the instructionsnecessary to perform fast convolution operations on the sampled IF GPSsignals to provide the pseudoranges.

In yet another embodiment of the present invention, the GPS receiver hasa common antenna for receiving GPS signals from in view satellites and aswitch for choosing between two signal paths. A first of the signalpaths provides for conventional GPS signal processing, whereinpseudoranges are calculated using correlation operations. Duringblockage conditions, a second signal path is used wherein the signalsare digitized and stored in memory for later processing. This laterprocessing is accomplished using fast convolution operations on thesampled GPS signals to provide the pseudoranges.

A further embodiment of the present invention provides a GPS receiverwith a common antenna for receiving GPS signals from in view satellitesand a common downconverter and digitizer. Sampled GPS signals receivedfrom the in view satellites are provided to a first signal path forconventional correlation processing to determine pseudoranges. Duringblockage conditions, the sampled GPS signals are provided to a secondsignal path for processing using fast convolution operations todetermine the pseudoranges. The two signal paths may be provided byseparate circuitry or by common circuitry executing computer readableinstructions appropriate for the given reception conditions.

An additional embodiment of the present invention provides a method fordetermining the position of a remote GPS receiver by storing GPSsatellite information, including Doppler, in the remote unit. Duringblockage conditions, the remote unit uses this information and sampledGPS signals from in view satellites to subsequently compute pseudorangesto the satellites using fast convolution operations. The computedpseudoranges may then used to determine the position of the remote unitThe position determination can occur at the remote unit or at abasestation. Where the position determination is performed at abasestation, the remote unit transmits the pseudoranges to thebasestation via a data link.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements and in which:

FIGS. 1A-1D are block diagrams of the major components of a remote GPSreceiving system utilizing the apparatus and methods of the presentinvention;

FIG. 2 is a block diagram of an exemplary embodiment of the dual modeGPS receiving system corresponding to FIG. 1C and utilizing theapparatus and methods of the present invention;

FIGS. 3A and 3B provide two alternatives for the RF and IF portions ofthe dual mode GPS receiver illustrated in FIG. 2;

FIG. 4 shows a flowgraph of the major software operations performed by aprogrammable DSP processor as illustrated in FIG. 2 in accordance withthe present invention; and

FIGS. 5A-5E illustrate signal processing waveforms at various stages ofprocessing in a dual mode GPS receiver according to the presentinvention.

DETAILED DESCRIPTION

This invention concerns apparatus and methods for computing the positionof a mobile, or remote, global positioning system (GPS) receiver withvery low received signal levels. As illustrated in FIGS. 1A-1D, a GPSreceiver 10 has first circuitry for receiving and processingpseudorandom sequences transmitted by a number of GPS satellites. Thefirst circuitry is configured to perform conventional correlationoperations on the received pseudorandom sequences to determinepseudoranges from the GPS receiver to the GPS satellites. Accordingly,the first circuitry is referred to herein as a conventional GPS receiver12. The GPS receiver 10 also includes second circuitry coupled to theconventional GPS receiver 12. The second circuitry is configured toreceive and process the pseudorandom sequences during blockageconditions. Blockage conditions are those conditions where theconventional GPS receiver 12 may have difficulty acquiring and/ortracking GPS signals from GPS satellites, such as occasions where theGPS signals have very low signal to noise ratios, urban canyonconditions where GPS signals are blocked due to tall buildings, tunnelsand other obstacles, conditions where the GPS receiver 10 is beingoperated under cover of trees or other foliage, in building applicationswhere GPS receiver 10 is being operated indoors, and other blockageconditions as will be appreciated by those skilled in the art.

The second circuitry is referred to herein as a snapshot GPS receiver 14which processes the pseudorandom sequences by digitizing and storing GPSdata made up of a predetermined record length of the receivedpseudorandom sequences. Once the GPS data has been stored, snapshot GPSreceiver 14 performs fast convolution operations on the stored data todetermine the pseudoranges. The manner in which these computations areperformed are discussed in detail below.

As illustrated in FIGS. 1A-1D, GPS receiver 10 includes two signalpaths, corresponding to conventional GPS receiver 12 and snapshot GPSreceiver 14. Various embodiments may include common circuitry withinthese signal paths for receiving GPS signals from in view satellites,downconverting the RF frequency of the received GPS signals to anintermediate frequency (IF) and/or digitizing the received GPS signals.For example, all the embodiments illustrated in FIGS. 1A-1D include acommon antenna 16 for receiving GPS signals. However, separate antennasfor conventional GPS receiver 12 and snapshot GPS receiver 14 could beused. The embodiment illustrated in FIG. 1B provides a common antennaswitch 18 for choosing between the two signal paths. During non-blockagereception conditions, switch 18 will allow received GPS signals to passfrom antenna 16 to conventional GPS receiver 12. Then, during blockageconditions, switch 18 will be configured to allow GPS signals to passfrom antenna 16 to snapshot GPS receiver 14. As shown in FIG. 1A,however, switch 18 may be omitted and signals from antenna 16 may beprovided simultaneously to both conventional GPS receiver 12 andsnapshot GPS receiver 14. For such an embodiment, conventional GPSreceiver 12 and snapshot GPS receiver 14 will communicate with oneanother (for example, through the use of a common processor or bypassing control information between separate processors) to determinewhich component will provide pseudorange computation.

An alternative embodiment, shown in FIG. 1C, provides a common RF to IFdownconverter 20 for both signal paths. This embodiment allows forreduced complexity in conventional GPS receiver 12 and snapshot GPSreceiver 14. The use of shared circuitry of this type also achieves aspace savings for the overall GPS receiver 10. It should be appreciatedthat although the embodiment of FIG. 1C includes switch 18, the sharedRF to IF converter 20 could also be used in the embodiment illustratedin FIG. 1A.

Yet another embodiment of GPS receiver 10 is illustrated in FIG. 1D. inthis embodiment, the shared circuitry includes RF to IF converter 20 anddigitizer 22. It will be appreciated that for this embodiment,conventional GPS receiver 12 is configured as a digital receiver, thatis, a receiver which computed pseudoranges using digital logic in theform of hardware correlators or a programmable signal processorexecuting appropriate instructions. Both types of conventional GPSreceivers are known in the GPS art. Further review of the detaileddiscussion of the signal processing functions performed by snapshot GPSreceiver 14 provided below will demonstrate how such an embodiment canbe implemented in accordance with the present invention.

The embodiment of FIG. 1D may have conventional GPS receiver 12 andsnapshot GPS receiver 14 as separate functional units (e.g., separateintegrated circuits) configured to communicate with one another.Alternatively, these two units may be formed on a single integratedcircuit 30 with shared circuitry configured to perform GPS signalprocessing appropriate to the reception conditions. That is, fornon-blockage conditions, the circuitry may be configured to performconventional GPS signal processing as described above. Then, whenblockage conditions are encountered, the circuitry could be configuredto perform snapshot GPS signal processing as will be described in detailbelow. Those skilled in the art will appreciate that these functionscould be accomplished with a programmable digital signal processor andappropriate computer readable instructions provided in, for example, aprogrammable read only memory (PROM) or with specially designed hardwarecircuitry. In either case, RF to IF downconverter 20 and digitizer 22may be included on the single integrated circuit 30. However, becausesuch components are commercially available in their separate form today,a presently preferred embodiment of the FIG. 1D implementation usesindependent RF to IF downconverter 20 and digitizer 22 as illustrated.

Those skilled in the art will appreciate that FIGS. 1A-1D merelyillustrate four of many potential embodiments of the present invention.Variations of these embodiments are possible wherein various circuitryor functions are shared between conventional GPS receiver 12 andsnapshot GPS receiver 14. These variations are within the spirit andscope of the present invention. For convenience, the embodiment of FIG.1C will be chosen for further detailed description below because itillustrates an embodiment with an intermediate level of shared circuitrywhich may be commercially available. It should further be noted thatpseudoranges may be used to compute the geographical position of GPSreceiver 10 in many different ways. Three examples are:

Method 1: By retransmitting the Satellite Data Messages to the GPSreceiver 10 from a basestation 50, GPS receiver 10 may combine thisinformation with the pseudorange measurements to compute its position.See, for example, U.S. Pat. No. 5,365,450, which is incorporated hereinby reference.

Method 2: GPS receiver 10 may gather the satellite ephemeris data fromthe reception of GPS signals in the normal manner that is commonlypracticed in the art using conventional GPS receiver 12. This data,which typically is valid for one to two hours, may be combined withpseudorange measurements from conventional GPS receiver 12 or, duringblockage conditions, from snapshot GPS receiver 14 to complete theposition calculation.

Method 3: GPS receiver 10 may transmit over a communications link 52 thepseudoranges computed by conventional GPS receiver 12 or snapshot GPSreceiver 14 to basestation 50 which can combine this information withthe satellite ephemeris data to complete the position calculation. See,for example, U.S. Pat. No. 5,225,842, which is incorporated herein byreference.

Method 2 provides the advantage of allowing GPS receiver 10 to operatein a self-contained manner. That is, no external communications arerequired. In Methods 1 and 3, basestation 50 requires informationregarding the satellites in view of GPS receiver 10. This may beaccomplished by knowing approximately die area in which GPS receiver 10is operating or by ensuring that basestation 50 and GPS receiver 10 havea common view of all satellites of interest and are positioned closeenough to one another to resolve a time ambiguity associated with therepetition rate of the GPS pseudorandom codes. This latter conditionwill be met for a range between basestation 50 and GPS receiver 10 of1/2 times the speed of light times the PN repetition period (1millisecond), or about 150 km.

Referring now to FIG. 2, one embodiment of GPS receiver 10 (thatcorresponding to the embodiment illustrated in FIG. 1C) is shown.Although the remaining discussion will be directed primarily to theembodiment illustrated in FIG. 2, it will be apparent to those skilledin the art that the apparatus and methods, including the signalprocessing functions to be described, may be used in any or all of theembodiments illustrated in FIGS. 1A-1D. For those embodiments, such asthe FIG. 1D embodiment, where different circuitry is shared betweenconventional GPS receiver 12 and snapshot GPS receiver 14, appropriatesignal paths would be provided.

Upon power up and initialization, switch 18 is configured to provide asignal path from RF to IF downconverter 20 to conventional GPS receiver12. Conventional GPS receiver 12 begins to compute pseudoranges usingconventional correlation operations as described above. Conventional GPSreceiver 12 also receives, decodes and stores satellite ephemeris datareceived from the in view GPS satellites. In addition, conventional GPSreceiver 80 stores Doppler information associated with each of the inview GPS satellites. This Doppler information will typically be in theform of frequency information and can be digitized and stored in RAM 32under the control of microprocessor 34. The Doppler information will beused by GPS receiver 10 when blockage conditions are encountered asdescribed below. Where available, conventional GPS receiver 12 may usedifferential GPS (DGPS) collection information transmitted frombasestation 50 to assist in determining the position of the GPS receiver10. The position so determined may be displayed on display 36 aslatitude and longitude information, a highlighted map display or anotherposition indication.

Alternatively, for an embodiment which utilizes the Method 3 approach asdescribed above, the pseudorange information computed by conventionalGPS receiver 12 can be transmitted to basestation 50 via modem 38 andcommunication antenna 40 for final position determination. In such anembodiment, the pseudorange information would be transmitted over acommunication link 42 from the GPS receiver 10 to the basestation 50.Basestation 50 would be configured with its own GPS receiver, therebyproviding a means of obtaining satellite ephemeris information. Aprocessor located at basestation 50 would be provided with informationregarding the approximate position of GPS receiver 10 (so that theproper satellite information could be selected) and would combine theephemeris information with the received pseudoranges to compute theposition of the GPS receiver 10. This computed position informationcould then be transmitted back to GPS receiver 10 via communication link42 where it would be displayed on display 36.

Under either method, GPS receiver 10 would display the results of aposition computation upon receipt of a fix command, i.e., a command todetermine the geographic location of GPS receiver 10. The fix commandmay be provided via a user using front panel controls 44 (or from thebasestation 50 when the optional modem 58 and associated communicationantenna 40 are used). Upon receipt of the fix command, microprocessor 34polls conventional GPS receiver 12 for a position report andconventional GPS receiver 12 demodulates the received GPS signals andproduces pseudorange information in the conventional fashion asdescribed above.

Under normal operating conditions, i.e., when antenna 16 has a clearview of the sky, conventional GPS receiver 12 is able to accuratelyacquire and track a sufficient number of GPS satellites to producepseudorange and/or position information for GPS receiver 10. However,such conditions may rapidly deteriorate when, for example, GPS receiver10 is moved into an urban or other canyon, inside a building, under atree or other foliage, or into some other area which results in at leasta partial blockage of the sky. Under these conditions, it is likely thatconventional GPS receiver 12 will be unable to track a sufficient numberof satellites to produce reliable pseudorange and/or positioninformation. The blockage conditions will result in a reduced signal tonoise ratio for the GPS signals received by conventional GPS receiver 12and these reduced signal to noise levels can be used as a trigger forGPS receiver 10 to switch to a "snapshot" mode.

Upon detecting the reduced signal to noise levels of received GPSsignals, or under user command entered through front panel controls 44,conventional GPS receiver 12 signals microprocessor 34 that it isencountering a blockage condition. Upon receipt of such a signal,microprocessor 34 configures GPS receiver 10 to operate in the snapshotmode. When the GPS receiver 10 enters the snapshot mode, microprocessor34 activates switch 18 so as to provide a signal path to the circuitrywhich makes up snapshot GPS receiver 14. This circuitry includes analogto digital converter (A/D) 46, digital snapshot memory 48, generalpurpose programmable digital signal processor (DSP) 52, program EPROM54, field programmable gate array (FPGA) 56, frequency synthesizer 58(which is also used in conjunction with conventional GPS receiver 12 toprovide a local oscillator for RF to IF downconverter 20), battery andpower control circuit 60 and microprocessor 34 (which may also controlthe operations of conventional GPS receiver 12).

While in snapshot mode, when GPS receiver 10 receives a fix command,microprocessor 34 activates A/D converter 46 and digital snapshot memory48 via the battery and power control circuit 60. This causes signalsfrom the GPS satellites, which are received via antenna 16 anddownconverted to an IF frequency in RF to IF downconverter 20, toundergo digitization. That is, the IF signals are sampled by A/Dconverter 46 at a frequency equal to the sample clock generated byfrequency synthesizer 58 and the resulting data is stored in digitalsnapshot memory 48. A contiguous set of such data, typically correspondsto a duration of 100 milliseconds to 1 second (or even longer). Theaddressing of digital snapshot memory 48 is controlled by FPGA 56.

Note that all this time (while the snapshot memory 48 is being filledwith the digitized GPS signals from the in view satellites) the DSP 52may be in a low power state. A/D converter 46 need only be turned on fora short period of time, sufficient to collect and store the datarequired for pseudorange calculation. After the data collection iscomplete, these converter circuits may be turned off, thus notcontributing to additional power dissipation during the actualpseudorange calculation. The pseudorange calculation is then performedusing, in one embodiment, a general purpose, programmable digital signalprocessing integrated circuit (DSP 52), as exemplified by a TMS320C30integrated circuit from Texas Instruments. DSP 52 is placed in an activepower state by the microprocessor 34 via the battery and power controlcircuit 60 prior to performing such calculations.

This DSP 52 differs from others used in some GPS units in that it isgeneral purpose and programmable, as compared to specialized customdigital signal processing integrated circuits. Furthermore, the DSP 52makes possible the use of fast convolution algorithms, which permit veryrapid computation of the pseudoranges by performing rapidly a largenumber of convolution operations between a locally generated referenceand the received GPS signals. Typically, 2046 such operations arerequired to complete the search for the epochs of each received GPSsignal. The fast convolution algorithms permit a simultaneous andparallel search of all such positions, thus speeding the requiredcomputation process by a factor of 10 to 100 over conventionalapproaches.

Once the DSP 52 has computed the pseudoranges (in the fashion describedin detail below), this information may be used to compute the positionof GPS receiver 10 using the satellite ephemeris data previously storedby conventional GPS receiver 12. The manner in which such positioncomputations are performed are well known in the art and the resultingposition information may be displayed on display 36 as latitude andlongitude (and altitude) information, as a highlighted map position orin another useful fashion. The position computations may be performed bymicroprocessor 34 executing, program commands stored in EEPROM 62 or byDSP 52 executing commands stored in Program EPROM 54. The positioncomputations may be made more accurately using DGPS corrections receivedfrom basestation 50 or other source of DGPS information (e.g., FMsubcarrier broadcasts).

Alternatively, for a Method 3-type embodiment, once the DSP 52 completesits computation of pseudoranges for each of the in view satellites, itmay transmit this information to basestation 50 across communicationlink 42 via modem 38 and under the control of microprocessor 34. At thistime the microprocessor 34 may cause the DSP 52 to again enter a lowpower state by sending an appropriate control signal to the battery andpower control circuit 60. In addition to the pseudorange data, a timetag may be simultaneously transmitted to basestation 50. The time tagindicates the elapsed time from the initial data collection in thedigital snapshot memory 48 to the time of transmission of the data overthe communication link 42. This time tag improves the capability of thebasestation 50 to complete the position calculation because it allowsthe computation of the GPS satellites respective positions at the timeof data collection.

Modem 38, in one embodiment, utilizes a separate communication antenna40 to transmit and receive messages over communication link 42. It willbe appreciated that modem 38 includes a communications receiver and acommunications transmitter which are alternatively coupled to antenna 42as required. Similarly, basestation 50 may use a separate antenna 64 totransmit and receive communication link messages, thus allowingcontinuous reception of GPS signals via GPS antenna 66 at thebasestation 50.

As indicated above, the digital snapshot memory 48 captures a recordlength of data corresponding to a relatively long period of time. Theefficient processing of this large block of data using fast convolutionmethods contributes to the ability of the present invention to processsignals at low received levels (e.g., when reception is poor due topartial blockage from buildings, trees, etc.). All pseudoranges forvisible GPS satellites are computed using this same buffered data. Thisprovides improved performance relative to continuous tracking (i.e.,conventional) GPS receivers in situations (such as urban blockageconditions) in which the signal amplitude is rapidly changing.

Where a communication link 42 is used, GPS receiver 10 may employ anautomatic frequency control (AFC) loop to lock to this carrier andthereby further calibrate its own reference oscillator. A messagetransmission time of 10 msec, with a received signal to noise ratio of20 dB, will normally allow frequency measurement via an AFC to anaccuracy of 10 Hz or better. This will typically be more than adequatefor the requirements of the present invention.

In one embodiment, the communication link 42 may be a commerciallyavailable narrow bandwidth radio frequency communications medium, suchas a two-way pager system. This system may be used in embodiments wherethe amount of data to be transmitted between the basestation 50 and theGPS receiver 10 is relatively small (e.g., where basestation 50 sends acommand to GPS receiver 10 to perform a position fix). In otherembodiments, where the amount of data to be transferred betweenbasestation 50 and GPS receiver 10 is relatively large, a higherbandwidth communication link 42 will be required.

A representative example of an RF to IF downconverter 20 and digitizingsystem for the GPS receiver 10 is shown in FIG. 3A (note that switch 18has not been shown for sake of clarity). The input signal from antenna16 at 1575.42 MHz is passed through a bandlimiting filter (BPF) 70 andlow noise amplifier (LNA) 72 and sent to a frequency conversion stage.The local oscillator (LO) 76 used in this stage is phase locked (via PLL78) to a 2.048 MHz (or harmonic thereof) temperature compensated crystaloscillator (TCXO) 80. In a preferred implementation, the LO frequencywould be 1531.392 MHz, which is 2991×0.512 MHz. The resulting IF signalis then centered at 44.028 MHz. This IF is desirable due to theavailability of low cost components near 44 MHz. In particular, surfaceacoustic wave filters (SAW), which are utilized in abundance intelevision applications, are readily available. Of course, otherbandlimiting devices could be used instead of SAW devices.

The received GPS signal is mixed with the LO signal in mixer 74 toproduce the IF signal. This IF signal is passed through a SAW filter 84,for precision bandlimiting to 2 MHz bandwidth, and then sent to an I/Qdown-converter 88, which translates the signal to near baseband (4 kHzcenter frequency nominally). The local oscillator frequency for thisdownconverter 88 is derived from the 2.048 MHz TCXO 80 as the 43rdharmonic of 1.024 MHz, that is 44.032 MHz.

The I/Q downconverter 88 is generally commercially available as an RFcomponent. It typically consists of two mixers and lowpass filters. Insuch instances, the input ports of one mixer are fed with the IF signaland the LO signal and the input ports to the other mixer are fed withthe same IF signal and the LO signal phase shifted by 90°. The outputsof the two mixers are lowpass filtered to remove feedthrough and otherdistortion products.

As shown in FIG. 3A, amplifiers 82 and 86 may he used before and afterthe bandlimiting operation as required.

The two outputs of the I/Q downconverter 88 are sent to two matched A/Dconverters 46 which sample the signals at 2.048 MHz. An alternativeimplementation replaces the A/D converters 46 with comparators (notshown), each of which outputs a two-valued (one-bit) sequence of data inaccordance with the polarity of the incoming signal. It is well knownthat this approach results in a loss of approximately 1.96 dB inreceiver sensitivity relative to a multilevel A/D converter. However,there may be substantial cost savings in use of a comparator vs. A/Dconverters, as well as in the reduced memory requirement in thefollowing digital snapshot memory 48.

An alternative implementation of the RF to IF downconverter anddigitizing system is shown in FIG. 3B which utilizes a bandpass samplingmethod (again, switch 18 has not been shown). The TCXO 80 employed is atfrequency 4.096 MHz (or an harmonic thereof). The TCXO 80 output may beused as the sample clock to the A/D converter 46 (or comparator); thisacts to translate the signal to 1.028 MHz. This frequency is thedifference between the 11th harmonic of 4.096 MHz and the input IFfrequency 44.028 MHz. The resulting 1.028 MHz IF is nearly one-fourththe sample rate, which is known to be nearly ideal in minimizingsampling type distortions. As compared to the I/Q sampling of FIG. 3A,this single sampler provides one channel of data rather than two, but attwice the rate. In addition, the data is effectively at an IF of 1.028MHz. I/Q frequency conversion to near 0 MHz would then be implemented bydigital means in the following processing to be described. The apparatusof FIGS. 3A and 3B are competitive in cost and complexity; oftencomponent availability dictates the preferred approach. It will beapparent to those skilled in the art, however, that other receiverconfigurations could be used to achieve similar results.

In order to simplify the following discussion, the following assumesthat the I/Q sampling of FIG. 3A is employed and that the digitalsnapshot memory 48 contains two channels of digitized data at 2.048 MHz.

Details of the signal processing performed in the DSP 52 may beunderstood with the aid of the flowgraph of FIG. 4 and the pictorial ofFIGS. 5A-5E. It will be apparent to those skilled in the art that themachine code, or other suitable code, for performing the signalprocessing to be described may be stored in EPROM 54. Other non-volatilestorage devices could also be used. The objective of the processing isto determine the timing of the received waveform with respect to alocally generated waveform. Furthermore, in order to achieve highsensitivity, a very long portion of such a waveform, typically 100milliseconds to 1 second, is processed.

In order to understand the processing, one first notes that eachreceived GPS signal (C/A mode) is constructed from a high rate (1 MHz)repetitive pseudorandom (PN) pattern of 1023 symbols, commonly called"chips." These "chips" resemble the waveform shown in FIG. 5A. Furtherimposed on this pattern is low rate data, transmitted from the satelliteat 50 baud. All of this data is received at a very low signal-to-noiseratio as measured in a 2 MHz bandwidth. If the carrier frequency and alldata rates were known to great precision, and no data were present, thenthe signal-to-noise ratio could be greatly improved, and the datagreatly reduced, by adding to one another successive frames. Forexample, there are 1000 PN frames over a period of 1 second. The firstsuch frame could be coherently added to the next frame, the result addedto the third frame, etc. The result would be a signal having a durationof 1023 chips. The phasing of this sequence could then be compared to alocal reference sequence to determine the relative timing between thetwo, thus establishing the so-called pseudorange.

The above process must be carried out separately for each satellite inview from the same set of stored received data in the digital snapshotmemory 48, since, in general, the GPS signals from different satelliteshave different Doppler frequencies and the PN patterns differ from oneanother.

The above process is made difficult by the fact that the carrierfrequency may be unknown by in excess of 5 kHz due to signal Doppleruncertainty and by an additional amount due to receiver local oscillatoruncertainty. These Doppler uncertainties are removed in one embodimentof the present invention by storing such information in RAM 32 asdescribed above. Alternatively, Doppler information could be transmittedfrom basestation 50, which simultaneously monitors all GPS signals fromin view satellites, in response to a signal via communication link 42indicating that GPS receiver 10 had entered the snapshot mode. Thus,Doppler search is avoided at the GPS receiver 10. The local oscillatoruncertainty can also be greatly reduced (to perhaps 50 Hz) by theaforementioned AFC operation performed using the communication link 42signal.

The presence of 50 baud data superimposed on the GPS signal still limitsthe coherent summation of PN frames beyond a period of 20 msec. That is,at most 20 frames may be coherently added before data sign inversionsprevent further processing gain. Additional processing gain may beachieved through matched filtering and summation of the magnitudes (orsquares of magnitudes) of the frames, as detailed in the followingparagraphs.

The flowgraph of FIG. 4 begins at step 100 with a command to initializea snapshot GPS processing operation (termed a "Fix Command" in FIG. 4).Where necessary (e.g., where no prior Doppler information has beenstored by conventional GPS receiver 12), the command includes atransmission from GPS receiver 10 to basestation 50 for Dopplerinformation for the in view satellites to be transmitted frombasestation 50 over communication link 42. At step 102, the GPS receiver10 computes its local oscillator drift, for example, by frequencylocking to the signal transmitted from the basestation 50. Analternative would be to utilize a very good quality temperaturecompensated crystal oscillator (TCXO 80) in the unit. For example,digitally controlled TCXOs, so-called DCXOs, currently can achieveaccuracy of about 0.1 parts per million, or an error of about 150 Hz forthe L1 GPS signal.

At step 104, microprocessor 34 activates switch 18; turns on power toA/D converters 46 and digital snapshot memory 48; and a snapshot of dataof duration K PN frames of the C/A code, where K is typically 100 to1000 (corresponding to 100 msec to 1 second time duration) is collected.When a sufficient amount of data has been collected, microprocessor 34turns off the A/D converters 46.

The pseudorange of each satellite is computed in turn as follows. First,at step 106 for the given GPS satellite signal to be processed, thecorresponding pseudorandom code (PN) is retrieved from EPROM 54. Asdiscussed shortly, the preferred PN storage format is actually theFourier transform of this PN code, sampled at a rate of 2048 samples perthe 1023 PN bits.

The data in digital snapshot memory 48 is processed in blocks of Nconsecutive PN frames, that is blocks of 2048N complex samples (N is aninteger typically in the range 5 to 10). Similar operations areperformed on each block as shown in the bottom loop (steps 108-124) ofFIG. 4. That is, this loop is performed a total of K/N times for eachGPS signal to be processed.

At step 108 the 2048N data words of the block are multiplied by acomplex exponential that removes the effects of Doppler on the signalcarrier, as well as the effects of drifting of the receiver localoscillator. To illustrate, suppose the Doppler frequency obtained fromconventional GPS receiver 12 (i.e., RAM 32) or the basestation 50 pluslocal oscillator offsets corresponded to f_(e) Hz. Then thepremultiplication of the data would take the form of the functione^(-j2)πf_(e) ^(nT),n= 0, 1, 2, . . . , 2048N-1!+(B-1)×2048 N, whereT=1/2.048 MHz is the sampling period, and the block number B ranges from1 to K/N.

Next, at step 110, the adjacent groups of N (typically 10) frames ofdata within the block are coherently added to one another. That is,samples 0, 2048, 4096, . . . 2048(N-1) -1 are added together, then 1,2049, 4097, . . . 2048(N-1) are added together, etc. At this point theblock contains only 2048 complex samples. An example of the waveformproduced by such a summing operation is illustrated in FIG. 5B for thecase of 4 PN frames. This summing operation may be considered apreprocessing operation which precedes the fast convolution operations.

Next, at steps 112-118, each of the averaged frames undergoes a matchedfiltering operation, which purpose is to determine the relative timingbetween the received PN code contained within the block of data and alocally generated PN reference signal. Simultaneously, the effects ofDoppler on the sampling times is also compensated for. These operationsare greatly speeded, in one embodiment, by the use of fast convolutionoperations such as Fast Fourier Transform (FFT) algorithms used in amanner to perform circular convolution, as presently described.

In order to simplify discussion, the above mentioned Dopplercompensation is initially neglected.

The basic operation to be performed is a comparison of the data in theblock being processed (2048 complex samples) to a similar reference PNblock stored locally. The comparison is actually done by (complex)multiplying each element of the data block by the corresponding elementof the reference and summing the results. This comparison is termed a"correlation." However, an individual correlation is only done for oneparticular starting time of the data block, whereas there are 2048possible positions that might provide a better match. The set of allcorrelation operations for all possible starting positions is termed a"matched filtering" operation. The full matched filtering operation isrequired in a preferred embodiment.

The other times of the PN block can be tested by circularly shifting thePN reference and performing the same operation. That is, if the PN codeis denoted p(0) p(1) . . . p(2047), then a circular shift by one sampleis p(1) p(2) . . . p(2047) p(0). This modified sequence tests todetermine if the data block contains a PN signal beginning with samplep(1). Similarly the data block may begin with samples p(2), p(3), etc.,and each may be tested by circularly shifting the reference PN andreperforming the tests. It should be apparent that a complete set oftests would require 2048×2048=4,194,304 operations, each requiring acomplex multiplication and addition.

A more efficient, mathematically equivalent method may be employed,utilizing the Fast Fourier Transform (FFT), which only requiresapproximately 12×2048 complex multiplications and twice the number ofadditions. In this method, the FFT is taken for the data block, at step112, and for the PN block. The FFT of the data block is multiplied bythe complex conjugate of the FFT of the reference, at step 114, and theresults are inverse Fourier transformed at step 118. The resulting dataso gotten is of length 2048 and contains the set of correlations of thedata block and the PN block for all possible positions. Each forward orinverse FFT operation requires P/2 log₂ P operations, where P is thesize of the data being transformed (assuming a radix-2 FFT algorithm isemployed). For the case of interest, P=2048, so that each FFT requires11×1024 complex multiplications. However, if the FFT of the PN sequenceis prestored in EPROM 54, as in a preferred embodiment, then its FFTneed not be computed during the filtering process. The total number ofcomplex multiplies for the forward FFT, inverse FFT and the product ofthe FFTs is thus (2×11+2)×1024=24576, which is a savings of a factor of171 over direct correlation. FIG. 4C illustrates the waveform producedby this matched filtering operation.

The preferred method of the current invention utilizes a sample ratesuch that 2048 samples of data were taken over the PN period of 1023chips. This allows the use of FFT algorithms of length 2048. It is knownthat FFT algorithms that are a power of 2, or 4, are normally much moreefficient than those of other sizes (and 2048=2¹¹). Hence the samplingrate so chosen significantly improves the processing speed. It ispreferable that the number of samples of the FFT equal the number ofsamples for one PN flame so that proper circular convolution may beachieved. That is, this condition allows the test of the data blockagainst all circularly shifted versions of the PN code, as discussedabove. A set of alternative methods, known in the art as "overlap save"or "overlap add" convolution may be utilized if the FFT size is chosento span a number of samples different from that of one PN frame length.These approaches require approximately twice the number of computationsas described above for the preferred implementation.

It should be apparent to one skilled in the art how the above processmay be modified by utilizing a variety of FFT algorithms of varyingsizes together with a variety of sample rates to provide fastconvolution operations. In addition, a set of fast convolutionalgorithms exist which also have the property that the number ofcomputations required are proportional to P log₂ P rather than P² as isrequired in straightforward correlation. Many of these algorithms areenumerated in standard references, for example, H. J. Nussbaumer, "FastFourier Transform and Convolution Algorithms," New York,Springer-Verlag, C1982. Important examples of such algorithms are theAgarwal-Cooley algorithm, the split nesting algorithm, recursivepolynomial nesting algorithm, and the Winograd-Fourier algorithm, thefirst three of which are used to perform convolution and the latter usedto perform a Fourier transform. These algorithms may be employed insubstitution of the preferred method presented above.

The method of time Doppler compensation employed at step 116 is nowexplained. In the preferred implementation, the sample rate utilized maynot correspond exactly to 2048 samples per PN frame due to Dopplereffects on the received GPS signal as well as local oscillatorinstabilities. For example, it is known that the Doppler shift cancontribute a delay error of ±2700 nsec/sec. In order to compensate forthis effect, the blocks of data processed in the above description needto be time shifted to compensate for this error. As an example, if theblock size processed corresponds to 5 PN frames (5 msec), then the timeshift from one block to another could be as much as ±13.5 nsec. Smallertime shifts result from local oscillator instability. These shifts maybe compensated for by time shifting the successive blocks of data bymultiples of the time shift required by a single block. That is, if theDoppler time shift per block is d, then the blocks are time shifted bynd, where n=0, 1, 2, . . . .

In general these time shifts are fractions of a sample. Performing theseoperations directly using digital signal processing methods involves theuse of nonintegral signal interpolation methods and results in a highcomputation burden. An alternative approach, that is a preferred methodof the present invention, is to incorporate the processing within thefast Fourier transform functions. It is well-known that a time shift ofd seconds is equivalent to multiplying the Fourier Transform of afunction by e^(-j2)πfd, where f is the frequency variable. Thus, thetime shift may be accomplished by multiplying the FFT of the data blockby e^(-j2)πnd/T f for n=0, 1, 2, . . . , 1023 and by e^(-j2)π(n-2048)d/Tf for n=1024, 1025, . . . , 2047, where T_(f) is the PN flame duration(1 millisecond). This compensation adds only about 8% to the processingtime associated with the FFT processing. The compensation is broken intotwo halves in order to guarantee continuity of phase compensation across0 Hz.

After the matched filtering operation is complete, the magnitudes, ormagnitudes-squared, of the complex numbers of the block are computed atstep 120. Either choice will work nearly as well. This operation removeseffects of 50 Hz data phase reversals (as shown in FIG. 5D) and lowfrequency carrier errors that remain. The block of 2048 samples is thenadded to the sum of the previous blocks processed at step 122. Step 122may be considered a post processing operation which follows the fastconvolution operation provided by steps 122-188. This continues untilall K/N blocks are processed, as shown by the decision block at step124, at which time there remains one block of 2048 samples, from which apseudorange is calculated. FIG. 5E illustrates the resulting waveformafter the summing operation.

Pseudorange determination occurs at step 126. A peak is searched forabove a locally computed noise level. If such a peak is found, its timeof occurrence relative to the beginning of the block represents thepseudorange associated with the particular PN code and the associatedGPS satellite.

An interpolation routine is utilized at step 126 to find the location ofthe peak to an accuracy much greater than that associated with thesample rate (2.048 MHz). The interpolation routine depends upon theprior bandpass filtering used in the RF/IF portion of the GPS receiver10. A good quality filter will result in a peak having a nearlytriangular shape with the width of the base equal to 4 samples. Underthis condition, following subtraction of an average amplitude (to removea DC baseline), the largest two amplitudes may be used to determine thepeak position more precisely. Suppose these amplitudes are denoted A_(p)and A_(p+1), where A_(p) ≧A_(p+1), without loss of generality, and p isthe index of the peak amplitude. Then the position of the peak relativeto that corresponding to A_(p) may be provided by the formula: peaklocation=p+A_(p/)(A_(p) +A_(p+1)). For example if A_(p) =A_(p+1), thenthe peak location is found to be p+0.5, that is, halfway between theindices of the two samples. In some situations the bandpass filteringmay round the peak and a three point polynomial interpolation may bemore suitable.

In the preceding processing, a local noise reference used inthresholding may be computed by averaging all the data in the finalaveraged block, after removing the several largest such peaks.

Once the pseudorange is found, the processing continues at step 128 in asimilar manner for the next satellite in view, unless all suchsatellites have been processed. Upon completion of the processing forall such satellites, the process continues at step 130 where thepseudorange data is transmitted to the basestation 50 over communicationlink 42, where the final position calculation of the remote is performed(assuming Method 3 is utilized). Alternatively, the position calculationmay be performed at GPS receiver 10 using satellite ephemeris datacollected by conventional GPS receiver 12 and stored in RAM 32. Thecomputed position may be displayed on display 36 and/or transmitted tobasestation 50 over communication link 42. Finally, at step 132, themajority of the snapshot GPS receiver circuitry of the GPS receiver 10is placed in a low power state, awaiting a new command to performanother positioning operation.

GPS receiver 10 can continue to operate in the snapshot mode, computingpseudoranges and/or positions periodically, until GPS receiver 10 ispositioned such that antenna 16 again has a clear view of the sky, atwhich time conventional GPS receiver 12 can again be used to acquire andtrack GPS satellites in order to obtain a position fix. In the foregoingembodiment, the processing of GPS signals from each satellite while inthe snapshot mode occurs sequentially in time, rather than in parallel.In an alternative embodiment, the GPS signals from all in-viewsatellites may be processed together in a parallel fashion in time.

Although the methods and apparatus of the present invention have beendescribed with reference to GPS satellites, it will be appreciated thatthe teachings are equally applicable to positioning systems whichutilize pseudolites or a combination of satellites and pseudolites.Pseudolites are ground based transmitters which broadcast a PRN code(similar to a GPS signal) modulated on an L-band carrier signal,generally synchronized with GPS time. Each transmitter may be assigned aunique PRN code so as to permit identification by a remote receiver.Pseudolites are useful in situations where GPS signals from an orbitingsatellite might be unavailable, such as tunnels, mines, buildings orother enclosed areas. The term "satellite", as used herein, is intendedto include pseudolite or equivalents of pseudolites, and the term GPSsignals, as used herein, is intended to include GPS-like signals frompseudolites or equivalents of pseudolites.

It will be further appreciated that the methods and apparatus of thepresent invention are equally applicable for use with the GLONASS andother satellite-based positioning systems. The GLONASS system differsfrom the GPS system in that the emissions from different satellites aredifferentiated from one another by utilizing slightly different carrierfrequencies, rather than utilizing different pseudorandom codes. In thissituation, substantially all the circuitry and algorithms describedabove are applicable, with the exception that when processing a newsatellite's emission, a different complex exponential multiplier is usedto preprocess the data. The operation may be combined with the Dopplercorrection operation of step 108 of FIG. 4, without requiring anyadditional processing operations. Only one PN code is required in thissituation, thus eliminating step 106.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. A global positioning system (GPS) receiver,comprising:first circuitry configured to be coupled to a GPS antenna,said first circuitry for receiving and processing pseudorandom sequencestransmitted by a plurality of satellites, said first circuitryconfigured to perform correlation operations on said pseudorandomsequences to determine pseudoranges from said GPS receiver to saidsatellites; and second circuitry coupled to said GPS antenna, saidsecond circuitry configured for receiving and processing saidpseudorandom sequences, said second circuitry configured to perform saidprocessing by digitizing and storing GPS data and by performing aplurality of convolution operations on a corresponding plurality ofblocks of said GPS data to provide a plurality of corresponding resultsof each convolution and summing a plurality of mathematicalrepresentations of said plurality of corresponding results to obtain aplurality of pseudoranges.
 2. A global positioning system (GPS) receiveras in claim 1 wherein said first circuitry and said second circuitrycomprise a single integrated circuit.
 3. A global positioning system(GPS) receiver as in claim 1 wherein said second circuitry comprises:amemory configured to receive and store said GPS data; and a programmabledigital signal processor (DSP) coupled to said memory, said programmableDSP configured to perform fast convolution operations on said GPS data.4. A global positioning system (GPS) receiver, comprising:an antenna forreceiving GPS signals at an RF frequency from in view satellites; adownconverter coupled to said antenna, said downconverter for reducingthe RF frequency of said received GPS signals to an intermediatefrequency (IF); a conventional GPS receiver adaptable to be coupled tosaid downconverter, said conventional GPS receiver for acquiring andtracking a plurality of GPS satellite signals and for extracting Dopplerinformation from said GPS satellite signals; and first circuitry coupledto said conventional GPS receiver and adaptable to be coupled to saiddownconverter, said first circuitry configured for receiving andprocessing said received GPS signals when said GPS receiver experiencesblockage conditions, said first circuitry configured to perform saidprocessing by digitizing and storing GPS data and by performing aplurality of convolution operations on a corresponding plurality ofblocks of said GPS data to provide a plurality of corresponding resultsof each convolution and summing a plurality of mathematicalrepresentations of said plurality of corresponding results to obtain aplurality of pseudoranges.
 5. A GPS receiver as in claim 4 wherein saidfirst circuitry comprises:a digitizer adaptable to be coupled to saiddownconverter upon receipt of a signal indicating that said conventionalGPS receiver is experiencing blockage conditions, said digitizer forsampling said IF GPS signals at a predetermined rate to produce sampledIF GPS signals; a memory coupled to said digitizer, said memory forstoring the sampled IF GPS signals; a digital signal processor (DSP)coupled to said memory, said DSP for performing fast convolutionoperations on said sampled IF GPS signals.
 6. A GPS receiver as in claim5 further comprising a local oscillator coupled to said downconverter,said local oscillator providing a first reference signal.
 7. A GPSreceiver as in claim 5 wherein said DSP compensates said sampled IF GPSsignals using said Doppler information and wherein said fast convolutionoperations provide a pseudorange information.
 8. A GPS receiver as inclaim 5 wherein said first circuitry further comprises a powermanagement circuit coupled to said digitizer, wherein after said IF GPSsignals are stored in said memory, said power management circuit powersdown said digitizer.
 9. A GPS receiver as in claim 5 wherein said GPSsignals originate from pseudolites.
 10. A GPS receiver as in claim 5further comprising a receiver coupled to said first circuitry, saidreceiver for receiving differential GPS correction signals from abasestation.
 11. A method for using a dual mode GPS receiver, saidmethod comprising the steps of:activating said GPS receiver in a firstmode of operation including,receiving GPS signals from in viewsatellites; downconverting and demodulating said GPS signals to extractDoppler information regarding in view satellites and to computepseudorange information; storing said Doppler information; detectingwhen said GPS receiver is experiencing blockage conditions andactivating a second mode of operation in response thereto, the secondmode including, digitizing said GPS signals at a predetermined rate toproduce sampled GPS signals; storing said sampled GPS signals in amemory; and processing said sampled GPS signals to derive pseudorangesby performing a plurality of convolution operations on a correspondingplurality of blocks of said sampled GPS signals to provide a pluralityof corresponding results of each convolution and summing a plurality ofmathematical representations of said plurality of corresponding resultsto obtain said pseudoranges.
 12. A method as in claim 11 wherein saidDoppler information is used during said second mode of operation tocompensate said sampled GPS signal.
 13. A method as in claim 12 whereinduring said first mode of operation, satellite ephemeris information isextracted from said GPS signals and stored in a memory and whereinduring said second mode of operation said satellite ephemerisinformation is applied to said pseudoranges to calculate a position ofsaid GPS receiver.
 14. A method as in claim 13 wherein said position isdisplayed to a user of said GPS receiver.
 15. A method as in claim 13wherein said position is transmitted from said GPS receiver to abasestation.
 16. A method as in claim 11 wherein said GPS signalsoriginate from pseudolites.
 17. A method as in claim 11 furthercomprising the steps of:receiving differential GPS correction signals atsaid GPS receiver before said step of processing; and using saiddifferential GPS signals during said step of processing to compute saidposition.
 18. A process utilizing global positioning system (GPS)satellites for determining the position of a remote sensor, the processcomprising the steps of:receiving GPS signals at said remote sensor froma plurality of in view GPS satellites; computing first pseudorangesutilizing said GPS signals and a conventional GPS receiver; computingfirst pseudoranges utilizing said GPS signals and a conventional GPSreceiver; utilizing said first pseudoranges and satellite ephemeris datato compute a geographic location for said sensor; detecting when saidremote sensor is experiencing blockage conditions; computing secondpseudoranges utilizing digitized and buffered segments of said GPSsignals, said computing comprising performing a plurality of convolutionoperations on a corresponding plurality of blocks of said bufferedsegments of said GPS signals to provide a plurality of correspondingresults of each convolution and summing a plurality of mathematicalrepresentations of said plurality of corresponding results to obtainsaid second pseudorange; and utilizing said second pseudoranges and saidsatellite ephemeris data to compute said geographic location.
 19. Theprocess of claim 18 wherein the step of computing second pseudorangesfurther comprises:storing said received GPS signals in a memory asstored data; processing said stored GPS signals for one or more of saidin view GPS satellites in a digital signal processor by, breaking saidstored data into a series of contiguous blocks whose durations are equalto a multiple of the frame period of the pseudorandom (PN) codescontained within the GPS signals; removing, for each block, the effectsof Doppler carrier and receiver local oscillator frequency uncertainty;creating, for each block, a compressed block of data with length equalto the duration of a pseudorandom code period by coherently addingtogether successive subblocks of data, said subblocks having durationequal to one PN frame; performing, for each compressed block, a matchedfiltering operation to determine the relative timing between thereceived PN code contained within the block of data and a locallygenerated PN reference signal, said matched filtering operationutilizing a fast convolution algorithm; determining said pseudorange byperforming a magnitude-squared operation on the products created fromsaid matched filtering operation and combining said magnitude-squareddata for all blocks into a single block of data by adding together saidblocks of magnitude-squared data to produce a peak, the location of saidpeak being determined using digital interpolation methods andcorresponding to said pseudorange.
 20. The process of claim 19 whereinsaid matched filtering operation comprises:performing a convolution ofthe compressed block's data against a stored replica of the pseudorandomsequence (PRS) of the GPS satellite being processed, said convolutionbeing performed using said fast convolution algorithms to produce aproduct of the convolution.
 21. The process of claim 19 wherein the fastconvolution algorithm used in processing the buffered GPS signals is aFast Fourier Transform (FFT) and the result of the convolution isproduced by computing the forward transform of said compressed block bya prestored representation of the forward transform of the pseudorandomsequence (PRS) of the GPS satellite being processed to produce a firstresult and then performing an inverse transformation of said firstresult to recover said result.
 22. The process of claim 18 wherein abasestation computes the geographic location of said sensor.
 23. Acomputer readable medium containing a computer program having executablecode for a global positioning system (GPS) receiver, said computerprogram comprising:first instructions for receiving GPS signals from inview satellites, said GPS signals comprising pseudorandom (PN) codes;second instructions for computing pseudoranges from said received GPSsignals using a conventional GPS receiver; third instructions fordetecting when said GPS receiver is experiencing blockage conditions anddigitizing said GPS signals at a predetermined rate to produce sampledGPS signals in response thereto; fourth instructions for storing saidsampled GPS signals in a memory; and fifth instructions for processingsaid sampled GPS signals, said fifth instructions comprising a matchedfiltering operation to determine the relative timing between said PNcodes and locally generated PN reference signals, said processingcomprising performing a plurality of convolutions on a correspondingplurality of blocks of said sampled GPS signals to provide a pluralityof corresponding results of each convolution and summing a plurality ofmathematical representations of said plurality of corresponding resultsto obtain a plurality of pseudoranges.
 24. A global positioning system(GPS) receiver apparatus, comprising:a first GPS receiver adaptable tobe coupled to a source of GPS satellite signals, said first GPS receiverfor acquiring and tracking a plurality of GPS satellite signals and forextracting Doppler information from said GPS satellite signals; and asecond GPS receiver adaptable to be coupled to said source of GPSsatellite signals during blockage conditions, said second GPS receiverconfigured to sample and store said GPS signals at a predetermined rateto produce sampled GPS signals and to process said sampled GPS signalsusing convolution operations, said second GPS receiver furtherconfigured to respond to a signal indicating that said GPS receiverapparatus is experiencing blockage conditions and to use saidconvolution operations wherein said second GPS receiver is coupled tosaid source of said GPS satellite signals and wherein said convolutionoperations comprises performing a plurality of convolutions on acorresponding plurality of blocks of said sampled GPS signals to providea plurality of corresponding results of each convolution and summing aplurality of mathematical representations of said plurality ofcorresponding results to obtain a plurality of pseudoranges.
 25. A GPSreceiver as in claim 24 wherein said first GPS receiver and said secondGPS receiver are formed on a single integrated circuit.
 26. A GPSreceiver as in claim 24 wherein said signal indicating that said GPSreceiver is experiencing blockage conditions is generated by said firstGPS receiver.
 27. A GPS receiver as in claim 24 wherein said GPS signalsoriginate from pseudolites.
 28. A GPS receiver as in claim 24 furthercomprising a communication receiver coupled to said first GPS receiverand to said second GPS receiver, said communication receiver forreceiving differential GPS correction signals.